Arthropod trackways Trackway Production

The trackway produced by an arthropod is a set of footprints repeated at an interval determined by the stride distance. Each individual footprint is known as a track, with a series of tracks being the trackway. Other marks may also be made in the substrate - an imprint is a mark left by a appendage other than the foot, for example a tail mark. An impression is a continuous imprint, such as a tail drag line. This is known as the medial impression if it occurs between the two sets of tracks. Together all these marks make up the trackway morphology.

If the producing arthropod has legs all of the same length and morphology the tracks will form two parallel lines. If legs are of different lengths or morphologies the tracks produced will fall at different distances from the mid-line. Analysis of this type of trackway enables us to estimate the number of legs of the producer; however we must proceed with caution. Behavioural differences will alter the morphology of the track, the gait employed may cause overprinting of one track onto another, or miss-steps. Undertrack fallout can occur when not all feet penetrate to the same depth in the sediment - if the trackway is exposed at a depth in the sediment lower than the original surface some of the tracks may be missing.

Other, preservational factors will influence trackway morphology. For example a track produced by a myriapod as it walks over firm, dry sediment will appear different to one produced as it walks over soft, wet sediment, even though the gait parameters may be the same. An example of the way that substrate type can influence trackway morphology is shown here:

Above is an example of the ichnogenus Diplichnites produced by a myriapod walking over fairly firm sediment. Two sets of parallel track groups can be seen, individual track clusters are clear.
This time the trace is made by the myriapod in a slightly wetter sediment. Individual tracks can no longer be seen, they grade into each other producing parallel 'tramlines'. This is the ichnogenus Diplopodichnus
Finally a myriapod trail produced in a very wet substrate. No evidence of tracks is preserved, instead a series of nested chevrons show where the myriapod's body has dragged over the surface, pulling the sediment with it. This trackway is called Miriandichnium

All these variations can cause confusion when trying to name a trackway or infer a producer. However they can also be valuable palaeoenvironmental or paleoecological indicators, for example a wet sediment trackway overprinted by a dry sediment trackway indicates a drying out sequence in the surrounding environment5.